Integrated magnetoresistive sensor, in particular three-axis magnetoresistive sensor and manufacturing method thereof

ABSTRACT

An integrated magnetoresistive device, where a substrate of semiconductor material is covered, on a first surface, by an insulating layer. A magnetoresistor of ferromagnetic material extends in the insulating layer and defines a sensitivity plane of the sensor. A concentrator of ferromagnetic material including at least one arm, extending in a transversal direction to the sensitivity plane and vertically offset to the magnetoresistor. In this way, magnetic flux lines directed perpendicularly to the sensitivity plane are concentrated and deflected so as to generate magnetic-field components directed in a parallel direction to the sensitivity plane.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage patent application based onInternational patent application number PCT/EP2011/074045, filed Dec.23, 2011, which claims the priority benefit of Italian patentapplication number TO10A001050, filed Dec. 23, 2010, which applicationsare hereby incorporated by reference to the maximum extent allowable bylaw.

BACKGROUND

1. Technical Field

The present disclosure relates to an integrated magnetoresistive sensor,in particular a three-axis magnetoresistive sensor, and to themanufacturing process thereof. In the following description, particularreference will be made to an anisotropic magnetoresistive (AMR) sensor,without, however, being limited thereto, and embodiments are applicablealso to other types of magnetoresistive sensors, such as thegiant-magnetoresistive (GMR) sensor and tunneling-magnetoresistive (TMR)sensor and other integrated magnetic-field sensors in themselvessensitive to magnetic fields parallel to the chip where they areintegrated.

2. Discussion of the Related Art

As is known, magnetoresistive sensors exploit the capacity ofappropriate ferromagnetic materials (called magnetoresistive materials,for example the material known by the name “permalloy”, formed by anFe—Ni alloy) to modify their resistance in the presence of an externalmagnetic field.

Currently, magnetoresistive sensors are obtained from magnetoresistivematerial strips. During manufacture, the magnetoresistive material stripcan be subjected to an external magnetic field so as to have apreferential magnetization in a preset direction (referred to as easyaxis), for example the longitudinal direction of the strip.

Before measuring the external magnetic field, a state of initialmagnetization along the axis of preferential magnetization is imposedvia a current pulse through a set/reset strap. In absence of externalmagnetic fields, the magnetization maintains the direction imposed bythe set/reset pulse, and the strip has maximum resistance in thisdirection. In presence of external magnetic fields having a directiondifferent from that of preferential magnetization, the magnetization ofthe strip changes, as does its resistance, as explained hereinafter withreference to FIG. 1.

In FIG. 1, a magnetoresistor 1 is formed by a magnetoresistive materialstrip having a longitudinal direction parallel to the axis X, whichforms also the direction of preferential magnetization. Themagnetoresistor 1 is traversed by a current I flowing in thelongitudinal direction of the strip. An external magnetic field Hy isdirected in a parallel direction to the axis Y and causes a rotation ofthe magnetization M through an angle α with respect to the current I. Inthis case we have

R=R _(min) +R _(d)cos²α

where R_(min) is the resistance of the magnetoresistor in case ofmagnetization M parallel to the axis Y (very high external magneticfield Hy), and R_(d) is the difference of resistance R_(max)−R_(min),where R_(max) is the resistance in case of magnetization directed in aparallel direction to the direction X.

For permalloy, the maximum ratio R_(d)/R is in the region of 2-3%.

Setting

${\sin^{2}\alpha} = {{\frac{{Hy}^{2}}{{Ho}^{2}}\mspace{14mu} {for}\mspace{14mu} {Hy}} \leq {Ho}}$

and

sin²α=1 for Hy≧Ho

where Ho is a parameter depending upon the material and the geometry ofthe strip 1, we have:

$\begin{matrix}{R = {{R_{m\; i\; n} + {{R_{d}\left\lbrack {1 - \left( \frac{Hy}{Ho} \right)^{2}} \right\rbrack}\mspace{14mu} {for}\mspace{14mu} {Hy}}} \leq {Ho}}} & (1)\end{matrix}$

FIG. 2 represents with a dashed line the plot of the resistance Rresulting from Eq. (1) (curve A).

It is moreover known, in order to linearize the plot of the resistance Rat least in an operative portion of the curve, to form, above themagnetoresistive material strip, transversal strips 2 (called “barberpoles”), of conductive material (for example aluminum), set at aconstant distance and with inclination of 45° with respect to thedirection X, as shown in FIG. 3.

In this situation, the direction of the current I changes, but not themagnetization. Consequently, Eq. (1) becomes:

$\begin{matrix}{R = {{R_{m\; i\; n} + {\frac{R_{d}}{2} \pm {\frac{R_{d}}{2}\left( \frac{Hy}{Ho} \right)\sqrt{1 - \left( \frac{Hy}{Ho} \right)^{2}}\mspace{14mu} {for}\mspace{14mu} {Hy}}}} \leq {Ho}}} & (2)\end{matrix}$

having a linear characteristic around the point Hy/Ho=0, as shown by thecurve B, represented by a solid line in FIG. 2.

In practice, in this neighborhood, the term under the square root isnegligible as compared to the linear term and thus we have

$\begin{matrix}{R = {R_{o} \pm {k\left( \frac{Hy}{Ho} \right)}}} & (3)\end{matrix}$

The ±sign in Eq. (3) depends upon the direction of the transversalstrips 2) (±45°.

FIG. 4 shows a magnetoresistive sensor 9 including four magnetoresistors1 having transversal strips 2 arranged in an alternating way. Themagnetoresistors 1 are connected so as to form a Wheatstone bridgeformed by two mutually parallel branches 3, 4 defining input terminals5, 6 and output terminals 7, 8. In detail, in each branch 3, 4, the twomagnetoresistors 1 a, 1 b have transversal strips 2 directed in anopposite direction (+45° and −45°, respectively). The magnetoresistors 1a, 1 b of one branch are arranged diametrically opposite to thecorresponding ones of the other branches (the magnetoresistor 1 a of thefirst branch 3 with transversal strips 2 at +45° is connected to thesecond input terminal 6 and the magnetoresistor 1 a of the second branch4 with transversal strips 2 at +45° is connected to the first inputterminal 5; the same applies to the magnetoresistors 1 b). A biasingvoltage Vb is applied between the input terminals 5, 6.

Trimmer resistors can be connected in series to each branch 3, 4, in away not shown, so that, in absence of an external magnetic fielddirected in a parallel direction to the direction of detection (here thefield Hx), the output voltage Vo across the output terminals 7, 8 iszero. Instead, in case of initial magnetization directed verticallydownwards, an external magnetic field Hx causes an increase in theresistivity of the magnetoresistors, here the straps 1 a, havingtransversal strips 2 directed at +45° and a corresponding reduction inthe resistivity of the other magnetoresistors 1 b having transversalstrips 2 directed at −45°. Consequently, each variation of resistancedue to an external field perpendicular to the magnetoresistors 1 a, 1 bcauses a corresponding linear variation of the output voltage Vo, thevalue of which thus depends in a linear way upon the external magneticfield Hx.

Because of the high sensitivity of magnetoresistive sensors of the typereferred to above, recently use thereof has been proposed for electroniccompasses in navigation systems. In this case, the external field to bedetected is represented by the Earth's magnetic field. To a firstapproximation, the Earth's magnetic field can be considered parallel tothe Earth's surface and the reading of the compass thus requires twosensors sensitive to the two directions of the plane locally tangentialto the Earth's surface.

Since, however, the inclination of the compass with respect to thetangential plane entails reading errors, to correct these errors threesensors are used, each having a sensitive axis directed according to thethree spatial axes X, Y, Z.

To this end, the three sensors are arranged with their sensitive axespositioned 90° with respect to each other. Whereas the production of asensor sensitive to fields directed in two directions does not createany difficulty, since they lie in the same plane, having the thirdsensor in the third direction involves a plane perpendicular to that ofthe first two sensors, as shown in FIG. 5, where the sensors X and Y areintegrated in a chip 10 and the sensor Z is integrated in a differentchip 11 and the chips 10, 11 are fixed to a same base or frame 12. Infact, in this case, the operations of assembly are much more complex andthe finished device is much more costly.

In addition, the alignment tolerances between the sensor Z and thesensors X and Y provided in different chips are greater than in case ofsensors integrated in a single chip so that a smaller precision isachieved as regards determination of the direction of the magneticfield, which is fundamental for the applications of an electroniccompass.

In addition, with the scaling down of the chips, the packages should beincreasingly small (e.g., from 5×5 mm² to 3×3 mm²); however, verticalassemblage is incompatible with the desired reduction.

The solutions proposed to the problem indicated are not, however,satisfactory. For example, patent application US 2009/0027048 describesa manufacturing process wherein a magnetoresistance is deposited in aV-shaped trench so that the sensitive layer is able to detect also partof the component perpendicular to the chip. On the other hand, thissolution renders more difficult deposition and definition of thetransversal strips or “barber poles”, of the metal interconnections, andof the auxiliary straps for calibration and for the set-reset procedure(the so-called “flipping”) for reduction of offset.

Similar problems exist also in case of a single sensor for detectingmagnetic fields directed perpendicularly to the horizontal plane, whenthe vertical arrangement of the device including the sensor is notpossible or when, even though the aim is to detect the horizontal fieldcomponents, it is necessary to arrange the device in a verticalposition.

SUMMARY

Embodiments provide a magnetoresistive sensor of an integrated type thatis able to detect external magnetic fields directed in a transversedirection to the magnetoresistive element plane.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments will now be described, purely by way of non-limitingexample, with reference to the attached drawings, wherein:

FIG. 1 shows a magnetoresistive element of a known type;

FIG. 2 shows the variation of resistance as a function of the fieldapplied for the elements of FIGS. 1 and 3;

FIG. 3 shows a known different magnetoresistive element;

FIG. 4 shows a magnetoresistive sensor in a Wheatstone-bridgeconfiguration;

FIG. 5 shows a known arrangement of elementary sensors for the detectionof magnetic fields along three Cartesian axes;

FIG. 6 shows the layout of an embodiment of the present magnetoresistivesensor;

FIG. 7 shows a cross-section through the sensor of FIG. 6;

FIG. 8 shows the layout of a different implementation of the presentsensor;

FIG. 9 shows a cross-section of the sensor of FIG. 8;

FIG. 10 shows the equivalent electrical circuit of the sensor of FIGS. 8and 9;

FIGS. 11 a-11 d are cross-sections through a wafer of semiconductormaterial in successive steps of fabrication of the sensor of FIGS. 8 and9;

FIGS. 12 and 13 are schematic illustrations of two possible variants ofthe present sensor;

FIG. 14 shows the block diagram of an electronic compass using thepresent magnetoresistive sensor;

FIG. 15 shows another embodiment of an electronic compass;

FIG. 16 shows the equivalent electrical circuit of the magnetoresistivesensors implemented in the electronic compass of FIG. 15, in anoperative condition;

FIG. 17 is a cross-section of another embodiment of the present sensor;

FIGS. 18 and 19 are cross-sections through wafers of semiconductormaterial in successive steps of fabrication of the sensor according to adifferent embodiment;

FIGS. 20 and 21 are cross-sections of yet other embodiments of thepresent sensor.

DETAILED DESCRIPTION

FIGS. 6 and 7 show a magnetoresistive sensor 15 formed in a chip 16comprising a substrate 17 of conductive material, for example silicon,and an insulating layer 18, for example, of silicon oxide, typicallyincluding a plurality of layers arranged on top of each other. Thesubstrate 17 has a front surface 19 covered by the insulating layer 18and a rear surface (back) 20. The front surface 19 and the rear surface20 extend parallel to the plane XY. At least one active area 24 may bepresent within the substrate 17 and may accommodate electroniccomponents 25, shown only schematically.

The insulating layer 18 accommodates a magnetoresistor 26, for examplean anisotropic magnetoresistor AMR, of a planar type, extending parallelto the plane XY and to the surfaces 19, 20 and thus defines asensitivity plane. In the example illustrated, the magnetoresistor 26 isformed by a plurality of magnetoresistive strips 27, for example ofpermalloy (Ni/Fe), connected at the ends by connection portions 28 so asto form a serpentine shape. Transversal strips 29 and connection lines30 are formed on top of the magnetoresistive strips 27; the transversalstrips 29 (called “barber poles”) are of conductive material (forexample aluminum), and the connection lines 30 connect themagnetoresistor 26 and the electronic components 25 together and to theoutside of the magnetoresistor. Here, the transversal strips 29, theconnection portions 28, and the connection lines 30 are formed in a samemetal level. In addition, other metal levels may be provided, connectedtogether by metal vias, in a per se known manner and not shown. A trenchor cavity 33 extends within the substrate 17, from the rear surface 20up to next or even as far as the front surface 19. The trench 33 isarranged laterally offset to the magnetoresistor 26 and accommodates aconcentrator 34 formed by a ferromagnetic material layer covering thesides and the bottom of the trench 33. The concentrator 34 is of a“soft” ferromagnetic material (i.e., one that can be magnetized easilyand does not maintain the magnetization after the external magneticfield has been removed). For example, a cobalt amorphous alloy or apermalloy can be used that is typically not anisotropic, or at leastwith easy axis not aligned to the vertical wall (axis Z).

In the cross-section of FIG. 7, the concentrator 34 has a U shape andcomprises two arms 34 a, 34 b and a base 34 c. The arms 34 a, 34 bcover, and extend contiguous to, the lateral walls of the trench 33,throughout the depth of the trench 33 (in the case shown, throughout thethickness of the substrate 17); the base 34 c covers, and is contiguousto, the bottom of the trench 33 and thus is here flush with the topsurface 19 of the substrate 17. As discussed below, the arms 34 a, 34 bhave a principal dimension along the axis Z much greater than theirthickness. As may be noted, both of the arms 34 a, 34 b are verticallyoffset to all the magnetoresistive strips 27.

Consequently, as represented in FIG. 7, which regards a cross-section inthe plane Y-Z, when the sensor 15 is subject to an external magneticfield H directed along the axis Z, the arms 34 a, 34 b of theconcentrator 34 cause deflection of the field lines H and generation ofa horizontal field component Hy directed along the axis Y and thusparallel to the sensitivity plane. The horizontal component Hy (andprecisely that generated as a result of the arm 34 a) can thus bedetected by the magnetoresistor 26 by means of an external readingcircuit in a known manner, since it causes a change of the magnetizationof the magnetoresistive strips 27 and thus of the resistance of themagnetoresistor 26.

In addition, since both the magnetoresistor 26 and the concentrator 34are of ferromagnetic material, a magnetic circuit is created that favorsthe concentration effect of the magnetic field and thus bestows highsensitivity on the sensor 15.

FIGS. 8-10 show an embodiment of the magnetoresistive sensor 15 that isinsensitive to magnetic fields directed in a parallel direction to theplane XY.

In detail, FIGS. 8 and 9 (in which parts corresponding to the sensor ofFIGS. 6, 7 have the same reference numbers) comprises fourmagnetoresistors 26 that form, respectively, resistors R1-R4, which arecoplanar and are connected so as to form a Wheatstone bridge 35 (FIG.10). To this end, the connection lines 30 connect first terminals of theresistors R1, R2 to each other and to a supply input 40 of the bridge35; first terminals of the resistors R3, R4 to each other and to agrounding input 41; second terminals of the resistors R1, R4 to a firstoutput terminal 42; and second terminals of the resistors R2, R3 to asecond output terminal 43. In practice, the resistors R1 and R4 form afirst branch of the bridge 35, and the resistors R2 and R3 form a secondbranch of the bridge 35, and the two branches are connected in parallelto each other and between the inputs 40 and 41, according to theequivalent electrical circuit of FIG. 10.

The resistors R1-R4 are the same as each other as regards thegeometrical and electrical characteristics of the magnetoresistivestrips 27 and of the connection portions 28, but have transversal strips29 inclined by ±45° with respect to the axis X. In particular, in theexample shown, the resistors R1-R4 are arranged symmetrically withrespect to an axis A parallel to the axis Y, where the resistors R1, R4of the first branch of the bridge 35 have transversal strips 29 directedat +45° with respect to the axis X and are arranged symmetrically withrespect to the resistors R2, R3 of the second branch of the bridge 35,having transversal strips 29 directed at −45° (+135°) with respect tothe axis X. In addition, the resistors R1-R4 are arranged symmetrically(apart from the direction of the transversal strips 29) about an axis Bparallel to the axis X. For the rest, each resistor R1-R4 is obtained inthe way described for the magnetoresistor 26 with reference to FIGS. 6,7.

In the example considered, the concentrator 34 extends longitudinallyand symmetrically with respect to the axis B so as to have the resistorsR1, R2 on a first side thereof and the resistors R3, R4 on the oppositeside. In addition (FIG. 9), the concentrator 34 extends between thepairs of resistors R1-R2 and R3-R4 so that the surfaces external to theU of the arms 34 a, 34 b are substantially aligned (but for tolerances)to the edges of the magnetoresistive strips 27 that face the axis B. Ingeneral, however, the arms 34 a, 34 b can be arranged at a distance fromthe resistors R1-R4 (along the axis Y); however, this distance isappropriately reduced and in any case kept less than 5 μm at worst.

With the configuration of FIG. 8, the arms 34 a, 34 b focus the magneticflow so as to deflect the lines of magnetic flow and create fieldcomponents parallel to the axis Y but with opposite directions, sincethey are guided through the ferromagnetic material of themagnetoresistors 26, following the path with minimum reluctance.Consequently, in the example of FIG. 9 with external magnetic field Hdirected in a direction Z and initial magnetization in a direction −X,the resistors R3, R4 beneath (in FIG. 9) the concentrator 34 see apositive field component Hy1, and the resistors R1, R2 above theconcentrator 34 see a negative field component Hy2, where Hy2=−Hy1.Thus, because of the different direction of the transversal strips 29,on the basis of Eq. (3), the resistance of the resistors R1, R3decreases by ΔR, whereas the resistance of the resistors R2, R4increases by ΔR. It follows that the output voltage V0 between theoutputs 42 and 43 is

Vo=Vb ΔR/Ro

i.e., proportional to the resistance variation and thus to the externalmagnetic field H. Consequently, a purposely provided reading circuit, onthe basis of the signal detected and of the geometrical configuration,is able to determine the amplitude of the external magnetic field H.

Instead, if the external magnetic field is of opposite sign, an outputvoltage

Vo=−Vb ΔR/Ro

is obtained, with opposite sign with respect to the previous one.

On the other hand, a possible magnetic field directed along the axis Y(for example, of positive sign) causes a same resistance change (e.g.,+ΔR) in the resistors R1 and R4, since they detect the same componentand have transversal strips 29 directed in the same direction. Inaddition, this field along the axis Y causes an equal resistance change,but with opposite sign (e.g., −ΔR), in the resistors R2 and R3. Itfollows that the output voltage V0 remains zero.

The sensor 15 of FIGS. 8, 9 is obtained as described hereinafter.

Initially (FIG. 11 a), a wafer 50 comprising the substrate 17, forexample of silicon, is subjected to the usual steps for forming thecomponents 25 (not shown in FIG. 11 a) within the active area 24. Then,after depositing a thin insulating layer (not shown separately) on thetop surface 19, the magnetoresistors 26 are formed with knowntechniques. For example, by means of a resist deposition and a standardphotolithography, a resist mask for the resistors is formed that coversthe entire wafer except for windows where the magnetoresistive strips 27are to be provided. Then, a thin film of magnetoresistive material, forexample permalloy, is deposited, the resist mask for the resistors isdissolved via solvents, and the metal above the resist mask is removed(lift-off technique), thus forming the magnetoresistive strips 27.Alternatively, it is possible to use dry or wet etching techniques.Next, a metal layer, for example of aluminum or copper, is deposited anddefined, to form the transversal strips (barber poles) 29, theconnection portions 28, and the interconnection lines 30. In this metallayer, possible trimmer resistors can be formed, as well as theset/reset strap for reducing the offset and possibly the calibrationstrap. Next, at least one dielectric layer (or more than one, if variousmetal levels are required) is deposited, thus completing the insulatinglayer 18.

Then (FIG. 11 b), the trench 33 is formed from the bottom surface 20 ofthe substrate. The trench 33 can be formed, for example, by means of adeep reactive ion etch (DRIE). In the example illustrated, the trench 33has vertical lateral walls, perpendicular to the rear surface 20, but itis also possible to provide a trench with inclined walls so as to forman angle of less than 90°, as represented with a dashed line on theright in FIG. 11 b.

The trench 33 can have a length equal to the thickness of the substrate17 or slightly smaller; for example, the length L can be greater than 50μm, typically L=300 μm or 500 μm. If the trench 33 does not extendthrough the entire thickness of the substrate 17, the distance D betweenthe bottom of the trench 33 and the top surface 19 of the substrate(and, to a first approximation, between the bottom of the trench 33 andthe magnetoresistive strips 27, given the thinness of the insulatinglayer underneath these) is kept as small as possible, e.g., smaller than30 μm, typically 0.5-10 μm. In fact, the smaller the distance D, thegreater the sensitivity of the sensor, since the gap between theconcentrator 34 and the magnetoresistors 26 represents an interruptionof the magnetic circuit where loss of some lines of flow may occur.

The width W of the trench 33 depends upon the aspect ratios that can beobtained with the used etching process. For example, with an aspectratio 1:20, in case of L=400 μm, W=20 μm; in case of L=500 μm, W=25 μm.In case of aspect ratio L/t=1:10, the minimum width may be equal to 12.5μm. In one embodiment, the width W can be approximately equal to thedistance between the mutually facing sides of two magnetoresistors 26arranged symmetrically with respect to the axis B.

Next (FIG. 11 c), a ferromagnetic layer 52 is deposited on the bottomsurface 20, for example by sputtering, and covers the lateral walls andthe bottom wall of the trench 33. The ferromagnetic layer 52 is of asoft ferromagnetic material, preferably a cobalt amorphous alloy orpermalloy, and may have a thickness comprised between 0.5 μm and 3 μm,for example 1 μm. If obtained by plating, the ferromagnetic layer 52 mayhave a larger thickness, for example of up to 10 μm.

Next (FIG. 11 d), the ferromagnetic layer 52 is defined, for example,via wet etching or alternatively by dry etching or lift-off so as toremove it from the bottom surface 20 of the substrate 17 and form theconcentrator 34. Then, the wafer 50 is diced, thus obtaining a pluralityof chips 16.

As an alternative to the above, the base 34 c of the concentrator 34 canbe removed (FIG. 12) since the part useful for concentration of themagnetic field and for closing the magnetic circuit is represented bythe arms 34 a, 34 b. It is moreover possible to remove also one of thetwo arms, for example the arm 34 b, as shown in FIG. 12. In this case,the remaining arm 34 a is positioned symmetrically with respect to themagnetoresistors 26, and the magnetoresistors 26 on opposite branches ofthe bridge 35 (FIG. 8) can be brought up closer to each other; forexample, they can be arranged at a distance linked to the process and tothe thickness of the layer that forms the concentrator 34. For instance,in the case of a front-back misalignment of 5 μm, the magnetoresistors26 can brought up to each other to a distance of approximately 10 μm,but in the case of a process with lower tolerances, also the distancebetween the magnetoresistors 26 can be further reduced, enabling asaving of area.

The sensor 15 with the concentrator 34 thus forms a magnetic circuitbending an external magnetic field directed perpendicularly (or having acomponent directed perpendicularly) to the magnetoresistors 26 so as togenerate parallel components that can be detected by themagnetoresistors. In addition, it concentrates the magnetic flow,increasing the sensitivity of the sensor. With the single-elementsolution of FIGS. 6-7, the magnetoresistor 26 remains sensitive to themagnetic fields parallel to the sensor 15 so that the single-elementsolution can typically be used in applications in which onlyperpendicular fields exist; instead, using the bridge solution 35 ofFIGS. 8-9 it is possible to eliminate external-field components parallelto the sensor.

By integrating, moreover, known magnetoresistive sensors 9 in the samechip 16 with the sensor 15, it is possible to obtain a three-axis AMR,GMR or TMR device having improved precision as compared tonon-integrated solutions, thanks to the reduction of mismatch of themagnetoresistors 26.

In addition, a saving of area and greater compactness of the three-axissensor is achieved.

The assembly of a single sensor Z sensitive to perpendicular fields orof the three-axis sensor moreover proves considerably simplified ascompared to the case of vertical assembly, as was necessary hitherto.

The concentrator is provided in a step of post-machining as compared toa standard AMR sensor and thus does not jeopardize or affect themanufacture of the others components of the sensor, including electroniccomponents integrated in the same chip for processing the signalsupplied by the magnetic sensor, thus not deteriorating substantiallythe reliability of the associated integrated circuits.

FEM (Finite Element Method) simulations conducted by the applicant havein effect shown that the sensor 15 has a sensitivity along the axis Zequal to or even greater than the sensitivity of the known sensor ofFIG. 4, if assembled in a vertical direction, with a same area or even areduction of the integration area.

Using the sensor 15 it is possible to provide a three-axis sensor forelectronic-compass applications.

For example, an electronic compass 60 can be obtained in a single chip16 by integrating two magnetoresistive sensors X and Y of a known type,without concentrator, rotated with respect to each other through 90°,alongside the sensor 15, as shown in FIG. 14.

Here, the electronic compass 60 comprises a first magnetoresistivesensor 61, detecting field components parallel to the axis X, a secondmagnetoresistive sensor 62, detecting field components parallel to theaxis Y, and the present magnetoresistive sensor 15 (the magnetoresistors26 of which are provided with a concentrator), detecting fieldcomponents parallel to the axis Z. Each of the magnetoresistive sensors61, 62 and 15 is connected to an own amplifier stage 63, which alsoeliminates the offset, and then to a calculation stage 64 determiningthe direction of the magnetic field in a per se known manner.

Alternatively, it is possible to use just two magnetoresistive sensors,of which at least one is built like the present magnetoresistive sensor15 provided with a concentrator, and moreover use a system of switchesfor changing the configuration of the bridge. For example, FIG. 15 showsan embodiment with two magnetoresistive sensors 15 a, 15 b, rotatedthrough 90° with respect to each other. Each magnetoresistive sensor 15a, 15 b comprises a first switch 66 arranged between the first resistorR1 and the first output terminal 42; and a second switch 67 arrangedbetween the second resistor R2 and the second output terminal 43. Inparticular, the first switch 66 has two positions: a first position inwhich the first switch 66 connects the first resistor R1 to the firstoutput terminal 42; and a second position, in which it connects thefirst resistor R1 to the second output terminal 43. Likewise, the secondswitch 67 has two positions: a first position, in which the secondswitch 67 connects the second resistor R2 to the second output terminal43; and a second position in which it connects the first resistor R1 tothe first output terminal 42.

The switches 66, 67 of the first magnetoresistive sensor 15 a arecontrolled by a same signal s1, and the switches 66, 67 of the secondmagnetoresistive sensor 15 b are controlled by a same signal s2, so thatthe two magnetoresistive sensors 15 a, 15 b can be controlledindependently. In this way, when the switches 66, 67 are in the firstposition, the corresponding magnetoresistive sensor 15 a, 15 b operatesin the way described above with reference to FIGS. 8-10, detecting thecomponent Z of an external field, whereas when the switches 66, 67 arein the second position (and thus the magnetoresistive sensors 15 a, 15 bhave the equivalent electrical circuit shown in FIG. 16), eachmagnetoresistive sensor 15 a, 15 b measures the respective planarcomponent (X and Y), being insensitive to the field component along theaxis Z.

In this way, a purposely provided control stage 70 integrated in thechip 16 can control the switches 66, 67 through the signals s1, s2 foracquiring first the planar components (X, Y) and then the perpendicularcomponents (Z), or vice versa or with any desired sequence.

Obviously, in the solution provided with switches 66, 67, thearrangement of the transversal strips 29 of the resistors R1, R2 of themagnetoresistive sensor 15 a and/or 15 b can be exchanged so that inthis sensor the magnetoresistors 26 is symmetrical with respect to theaxis B, instead of the axis A. In this case, in practice, the firstbranch of the bridge would be formed by the magnetoresistors R2 and R4,and the second branch of the bridge would be formed by themagnetoresistors R1 and R3. Consequently, with the arrangement of theswitches 66, 67 represented with a solid line in FIG. 15, themagnetoresistive sensors 15 a, 15 b detect components of magnetic fielddirected, respectively, parallel to the axis X and to the axis Y and areable to detect components of magnetic field directed in a paralleldirection to the axis Z when the switches 66, 67 are in the positionrepresented by the dashed line.

In addition, as has been mentioned, just one of the two magnetoresistivesensors 15 a, 15 b may be provided (detecting alternatively thedirection Z and one direction between X and Y) and the others beingprovided without concentrator and without switches but being rotatedthrough 90° so as to detect the other between the directions X and Y.

Finally, it is clear that modifications and variations may be made tothe sensor described and illustrated herein, without thereby departingfrom the scope of the present invention, as defined in the annexedclaims.

For example, the magnetoresistors 26 can be provided in a different way,by a single segment or by shaping the ferromagnetic material so as toalready have a serpentine shape; more than four magnetoresistors may beused, and/or more than one concentrator could be provided; for example,another set of magnetoresistors 26 with an own concentrator 34 could bearranged alongside the elements shown in FIG. 9. In this case, the setsof magnetoresistors-concentrators would have to be arranged at adistance such as not to influence each other and could be connected soas to form in any case a Wheatstone bridge 35, the resistors R1-R4whereof are formed by pairs of resistors 26 connected in series. In thiscase, since the magnetic field concentrated on the magnetoresistivestrips 27 making up each magnetoresistor 26 decreases sensibly with thedistance thereof from the edge of the concentrator 34, by dividing eachmagnetoresistor 26 into a number of parts, it is possible to increasethe sensitivity at the expense of a greater occupation of area.

In addition, the concentrator could be provided on the front of thechip, for example in a different chip bonded to the insulating layer 18.Such a solution is e.g. shown in FIG. 17, wherein a second substrate 120of conductive material, for example silicon, has been bonded to firstsubstrate 117 through bumps 110. The second substrate 120 has a trench133 facing the insulating layer 18 of the first substrate 17 and aconcentrator 134, of a ferromagnetic material layer, covers the sidesand the bottom of the trench 133, analogously to the concentrator 34 ofFIG. 9. In this case, the first and second substrates 17, 117 are workedindependently to form the active area 24 and any components in the firstsubstrate 17 and to form the concentrator 134 in the second substrate117. To this end, the steps described with reference to FIGS. 11 b-11 dare performed on the second substrate 117 before bonding. Then, thesecond substrate 117 is bonded to the first substrate, so have theconcentrator 134 laterally offset to the magnetoresistor 26 and facingthe insulating layer 18 of the first substrate 17.

The solution of FIG. 17 may allow a better alignment of the concentrator134 and does not require post-processing steps.

According to another embodiment, the trench and the concentrator areformed before forming the insulating layer and the magnetoresistor fromthe top surface 19. An embodiment of such a solution is shown in FIGS.18, 19. In this case, see FIG. 18, the first substrate 217 is etched soas to form a trench 233, the trench 233 is coated with a ferromagneticlayer, which is then defined to form a concentrator 234, analogously towhat described with reference to FIGS. 11 a-11 d. If not completelyfilled by the ferromagnetic layer, the trench 233 is here filled by aninsulating layer 235. Then, FIG. 19, the magnetoresistors 26 and theconnection lines 30 are formed and the insulating layer 218 iscompleted, as described above.

In yet another embodiment, shown in FIG. 20, the concentrator is formedin the insulating layer. In this case, after forming themagnetoresistors 26 and the connection lines 30 and completing theinsulating layer 318, the latter is etched to form a trench 333, aferromagnetic layer is deposited and defined to form a concentrator 334(here in the shape of two arms, analogously to FIG. 12. In practice, thearms of the concentrator 334 extend from the free surface of theinsulating layer 318 down to in proximity of the magnetoresistors 26.The layout is the same of FIG. 8, as for the embodiments of FIGS. 17-19.

This solution has low manufacturing costs and good alignmentcharacteristics.

In FIG. 21, a first concentrator 434 is formed in the substrate 417, andsecond concentrators 435 are formed in the insulating layer 418. In thiscase, the first concentrator 434 is the same as the concentrator 34 ofFIG. 12, including two arms within a trench 433 and arrangedsymmetrically to the magnetoresistors 26. The second concentrators 435extend from the free surface of the insulating layer 418 down to next tothe magnetoresistors 26, symmetrically thereto, at the external borderthereof.

In FIG. 21, the second concentrators may be obtained by forming thintrenches in the insulating layer 418 and filling them with softferromagnetic material, that may be the same of the first concentrator434.

In this way, as shown by the arrows 400, the magnetic lines are betterguided in the horizontal direction in the area of the magnetoresistors26 and are collected by the second concentrators 435, ensuring a betterconcentration of the magnetic field, thus increasing the efficiency ofthe system.

This same solution could be applied to the embodiment of FIGS. 7, 9, 13,and 19. Additionally, second concentrators may be formed in the firstsubstrate 117 of FIG. 17 or in the substrate 317 of FIG. 20.

As has been mentioned, the walls of the trench 33 could be inclined, byeven as much as 45°.

In addition, the solution of FIG. 15, with sensors 15 a, 15 b providedwith switches 66, 67 can also be used for three-dimensional devices fordetecting magnetic fields with different applications.

The various embodiments described above can be combined to providefurther embodiments. These and other changes can be made to theembodiments in light of the above-detailed description. In general, inthe following claims, the terms used should not be construed to limitthe claims to the specific embodiments disclosed in the specificationand the claims, but should be construed to include all possibleembodiments along with the full scope of equivalents to which suchclaims are entitled. Accordingly, the claims are not limited by thedisclosure.

Such alterations, modifications, and improvements are intended to bewithin the spirit and scope of the invention. Accordingly, the foregoingdescription is by way of example only and is not intended as limiting.The invention is limited only as defined in the following claims and theequivalents thereto.

What is claimed is:
 1. An integrated magnetoresistive device,comprising: a substrate having first and second surfaces, an insulatinglayer extending on the first surface, a first magnetoresistor of a firstferromagnetic material extending in the insulating layer and having asensitivity plane, and a concentrator of a second ferromagnetic materialincluding a first arm extending longitudinally in a transversaldirection to the sensitivity plane and vertically offset to the firstmagnetoresistor, the concentrator being configured to deflect magneticflux lines directed perpendicularly to the sensitivity plane and togenerate magnetic field components directed in a parallel direction tothe sensitivity plane.
 2. An integrated magnetoresistive deviceaccording to claim 1, wherein the substrate is a semiconductor substrateand the concentrator is formed in the substrate.
 3. An integratedmagnetoresistive device according to claim 1, wherein the substrate is asemiconductor substrate and has a trench extending from the secondsurface toward the first surface and the first arm of the concentratoris adjacent to and covers a lateral wall of the trench.
 4. An integratedmagnetoresistive device according to claim 1, wherein the substrate hasa trench extending from the first surface toward the second surface andthe first arm of the concentrator is adjacent to and covers a lateralwall of the trench.
 5. An integrated magnetoresistive device accordingto claim 1, comprising a ferromagnetic collector in the insulatingmaterial arranged offset with respect to the magnetometer on an oppositeside thereof with respect to the first arm.
 6. An integratedmagnetoresistive device according to claim 1, wherein the concentratoris formed in a trench in a body bonded to the insulating layer with thetrench facing the insulating layer.
 7. An integrated magnetoresistivedevice according to claim 1, wherein the concentrator is formed in atrench in the insulating layer.
 8. An integrated magnetoresistive deviceaccording to claim 1, wherein the concentrator includes a pair ofdistinct arms including the first arm, each longitudinally extendingtransversely to the sensitivity plane.
 9. An integrated magnetoresistivedevice according to claim 1, wherein the concentrator has a U shape in across-section, and comprises a pair of distinct arms, including thefirst arm, connected by a base portion substantially parallel to thefirst surface.
 10. An integrated magnetoresistive device according toclaim 1, wherein the first arm of the concentrator has a thicknesscomprised between 0.5 and 10 μm, for example 1 μm and a length equal orgreater than ten times the thickness, and the distance between the armand the first magnetoresistor in a perpendicular direction to thesensitivity plane is smaller than 30 μm.
 11. An integratedmagnetoresistive device according to claim 1, wherein the secondferromagnetic material is selected between a cobalt alloy and a Fe—Nialloy, for example of “permalloy”.
 12. An integrated magnetoresistivedevice according to claim 1, comprising a second magnetoresistorextending in the insulating layer and coplanar to the firstmagnetoresistor, wherein the concentrator is equidistant from the firstand the second magnetoresistors.
 13. An integrated magnetoresistivedevice according to claim 1, comprising a first plurality ofmagnetoresistors, including the first magnetoresistor, connected to forma first Wheatstone bridge including a first and a second branch mutuallyconnected in parallel between a pair of input terminals and defining apair of taps forming output terminals, wherein: the magnetoresistors ofthe first branch are arranged symmetrically to the magnetoresistors ofthe second branch with respect to a first axis parallel to thesensitivity plane; the magnetoresistors each include a ferromagneticmaterial strip extending longitudinally parallel to a second axissubstantially perpendicular to the first axis and parallel to thesensitivity plane and a plurality of conductive transversal stripsoverlaid to the respective ferromagnetic material strip; the conductivetransversal strips extend transversally to the first and to the secondaxes, parallel to the sensitivity plane; the concentrator extendslongitudinally parallel to the second axis; the ferromagnetic materialstrips are arranged symmetrically with respect to the concentrator; andfor each branch, the conductive transversal strips of themagnetoresistors of the branch extend parallel to each other.
 14. Anintegrated magnetoresistive device according to claim 13, comprising asecond plurality of magnetoresistors connected to form a secondWheatstone bridge, wherein the magnetoresistors of the second pluralityeach include a ferromagnetic material strip, the ferromagnetic materialstrips of said magnetoresistors of said second plurality are arrangedperpendicularly to the ferromagnetic material strips of themagnetoresistors forming the first Wheatstone bridge.
 15. An integratedmagnetoresistive device according to claim 14, of a three-axis type,wherein the first Wheatstone bridge comprises a first and a secondswitches arranged respectively on the first and on the second branchesand configured to, in a first operative condition, respectively connectthe first magnetoresistor to the first output terminal and the secondmagnetoresistor of the second branch to the second output terminal and,in a second operative condition, respectively connect the firstmagnetoresistor to the second output terminal and the secondmagnetoresistor of the second branch to the first output terminal. 16.An integrated electronic compass, comprising a magnetoresistive deviceaccording to claim 13, and a count unit, coupled to the magnetoresistivesensors and configured to calculate an angle of a component of magneticfield parallel to said sensitivity plane.
 17. A process formanufacturing an integrated magnetoresistive device, comprising thesteps of: forming an insulating layer on top of a first surface of asubstrate having first and second surfaces; forming a magnetoresistor ofa first ferromagnetic material in the insulating layer, themagnetoresistor defining a sensitivity plane; and forming a concentratorof a second ferromagnetic material including forming a first armextending longitudinally in a transverse direction to the sensitivityplane and vertically offset with respect to the magnetoresistor.
 18. Aprocess according to claim 17, wherein the step of forming aconcentrator comprises forming a trench in the substrate and coveringwalls of the trench with a layer of the second ferromagnetic material.19. A process according to claim 18, wherein forming a trench comprisesetching the substrate from the second surface to next to the firstsurface.
 20. A process according to claim 18, wherein forming a trenchcomprises etching the substrate from the first surface toward the secondsurface and forming the concentrator before forming the insulatinglayer.
 21. A process according to claim 17, comprising forming aferromagnetic collector in the insulating material arranged offset withrespect to the magnetometer on an opposite side thereof with respect tothe first arm.
 22. A process according to claim 17, wherein forming theconcentrator comprises forming a trench in a body distinct from thesubstrate and bonding the body to the insulating layer with the trenchfacing the insulating layer.
 23. A process according to claim 17,wherein forming the concentrator comprises forming a trench in theinsulating layer.
 24. A process according to claim 17, wherein formingthe concentrator includes forming a trench, covering walls of the trenchwith a layer of the second ferromagnetic material, and selectivelyremoving the layer of the second ferromagnetic material from the secondsurface so that the concentrator has a U shape in cross-section.
 25. Aprocess according to claim 17, wherein forming the concentrator includesforming a trench, covering walls of the trench with a layer of thesecond ferromagnetic material, selectively removing the layer of thesecond ferromagnetic material from the second surface and from the baseof the trench so as to form two arms of the concentrator coveringlateral walls of the trench.